Drag reduction using riblets downstream of a high Reynolds number inclined forward step flow

Micro-riblet is an efficient passive method for controlling turbulent boundary layers, with the potential to reduce frictional drag. In various applications within the transportation industry, flow separation is a prevalent flow phenomenon. However, the precise drag reduction performance of riblets in the presence of flow separation remains unclear. To address this, an inclined forward step model is proposed to investigate the interaction between riblet and upstream flow separation. The large eddy simulation (LES) method is applied to simulate the flow over geometries with different step angles and riblet positions. The results show riblets still reduce wall frictional resistance when subjected to the upstream flow separation. Remarkably, as the angle of the step increases from 0 degrees to 30 degrees, the drag reduction experiences an increment from 9.5% to 12.6%. From a turbulence statistics standpoint, riblets act to suppress the Reynold stress in the near-wall region and dampen ejection motions, thus weakening momentum exchange. Quadrant analysis reveals that with the augmentation of flow separation, the Q2 motion within the flow field intensifies, subsequently enhancing the riblet-induced drag reduction. Moreover, the position of the rib lets has a significant impact on the pressure drag. Riblets close to the point of separation enhance flow separation, altering the surface pressure distribution and thus increasing the resistance. The results reveal that when the riblets are positioned approximately 160 riblet heights away from the step, their effect on the upstream flow separation becomes negligible. The precise performance of riblets under complex flow conditions is important for their practical engineering application

基于卢比变换码的分解重组编码算法

LT（卢比变换）码是第一个能够实际应用的数字喷泉码。尽管采用BP（置信）译码算法的LT码的编译码复杂度已接近线性,但依然会增加较大的时延。文章提出了一种DRE（分解重组编码）算法,对所有信息包分解后重新组合,分成多组并行编码,有效地降低了编码时间和译码的操作数,减少了数据传输时延。该编码方法可适用于不同度分布函数。仿真结果表明,当信息包数量K=1 000和2 000时,采用同样的度分布可以达到NRE（非重复编码）的性能,并能够减少约75%的编码时间和大量的译码操作数

Influence of rectangular strips size on aerodynamic performance of a high-speed train subjected to crosswind

Conventional studies usually assume that the train surface is smooth, so as to simplify the numerical calculation. In fact, the surface of the train is irregular, which will change the flow characteristics in the boundary layer and further affect the aerodynamic performance of a train. In this work, roughness is applied to the roof of a 1:25 scaled train model in the form of longitudinal strips. Firstly, the improved delayed detached eddy simulation (IDDES) method is adopted to simulate the aerodynamic performance of the train model with both smooth and rough surface, which are subjected to crosswind. Results show that the side force coefficient and the roll moment coefficient subjected to rough model decreased by 3.71% and 10.56% compared with the smooth model. Then, the width, height and length of the strips are selected as variables to design different numerical simulation schemes based on the orthogonal experimental design method. Through variance analysis, it can be found that four design parameters have no significant effect on the side force coefficient. Meanwhile, for the roll moment coefficient, the length of the strips in the straight region of the train has a significant effect and the width of the strips has a highly significant effect on it. These conclusions can provide a theoretical basis to improve the aerodynamic performance of the high-speed train subjected to crosswind. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved

Dechirp-based method for synthesizing high resolution range profile(HRRP) via stepped-frequency waveforms

研究了一种基于子脉冲去斜的高分辨距离像合成新方法。首先,用本地参考信号对接收的步进频脉冲串信号进行去斜处理,然后把去斜之后的信号按顺序在时域上拼接起来得到宽带回波,最后按传统的处理方法得到目标的一维距离像和二维逆合成孔径雷达像。这种方法避免了传统时域拼接法的上采样、频移和相位补偿运算,减小了处理复杂度,易于工程应用。A novel method for synthesizing the high resolution range profile(HRRP) with respect to the stepped frequency waveforms,which is based on dechirp processing,is proposed.First,the received stepped-frequency pulses are dechirped by local reference signals.The wideband echoes are obtained by joining the dechirped pulses in order in time-domain.Finally,the 1-D HRRP and the 2-D inverse synthetic aperture radar(ISAR) images can be obtained according to the traditional processing.The up sampling,frequency shift and phase compensation operations are avoided with this new method,thus the processing complexity is reduced and this new method is easier to implement in radar systems

Multi-objective design optimization of the combinational configuration of the upstream energy deposition and opposing jet for drag reduction in supersonic flows

Optimization design has been widely used in the supersonic vehicle design process and the drag reduction characteristic is an important objective of the optimization. The drag reduction mechanism applied to the blunt body with the combinational configuration of the upstream energy deposition and opposing jet for drag reduction has been conducted numerically. In the current study, the three-dimensional coupled implicit compressible Reynolds Averaged Navier-Stokes equations and Menter's shear stress transport turbulence model are employed to simulate the flow fields around the blunt body with the combined method. The results show that in the jet-to-freestream total-pressure ratio of 0.2 and 0.4, the drag is reduced by 47.44% and 45.96%, respectively. Further, the Latin hypercube method is used for the generation of initial samples for optimization and the multi-objective design optimization algorithm coupled with the Kriging model surrogate model is applied to determine optimal flow control parameters. The drag reduction factor R-d and drag reduction effectiveness E-eff are selected as optimization objectives. The Pareto-optimal front for the multi-objective design optimization results is acquired and there exists a challenging tradeoff between the two optimization objectives. The drag reduction factor R-d and drag reduction effectiveness E-eff further increase as much as 28.16% and 116.47%, respectively. The jet has a stronger penetration in the optimum design condition, and the findings suggest that the strategy of adding energy spot to the upstream flow field of the opposing jet can be an effective way for drag reduction. (C) 2020 Elsevier Masson SAS. All rights reserved

基于预测滤波的卫星交会对接相对位姿确定方法

基于视频制导系统提供的视线测量,引入一种高精度、高鲁棒性 的、实时的预测滤波算法,建立系统的误差模型,对角速度和位移速度进行一步预测,从而精确地估计卫星的相对位置和相对姿态。此外提出修正方案,通过卡尔曼 增益对相对姿态和相对位移进行修正和更新,估计性能得到改进优化。通过卫星交会对接仿真实验验证算法的有效性,结果表明,预测滤波算法高精确估计相对姿态 和相对位置,修正后算法得到了更高的精度结果,满足卫星交会对接要求,对在轨实验有重要参考价值